Hesititation near full throttle?

By slowxr, Posted January 12, 2004

I've got a new uncorked 650. I just noticed that at full throttle the bike pulls hard and clean but when I back off the throttle just a little, the bike hesitates. The bike runs great from closed throttle to wide open. It's just this small area that I'm experiencing problems.

What is this caused by?

By Grimmer, Posted January 12, 2004

Thats one of the ways you check to see if your main jet is sized properly. When you close down slightly on the throttle you are forcing a lean condition. How your bike responds tells you a good deal. I forget honestly, it has been a year since I jetted mine. Search around for jetting information. Are you running a 172, 175? Main jet is where I would look first.

By qadsan, Posted January 12, 2004

Hmmm...

A couple of things come to my mind that might be suspect to check out.

1) Intake leak. This is a classic sign of an intake leak. When the bike is at WOT, the air/fuel mixture is easily sucked through the intake manifold with little restriction (low vacuum). When the throttle is chopped, the throttle slide blocks / restricts the air flow and the intake manifold vacuum greatly increases. If the leak is small enough (loose intake boot), then the higher vacuum will suck in the air and lean out the mixture causing the bike to hesitate. When you're back on the throttle, the vacuum decreases and won't stress the boot as much (air will take the path of the least resistance which is through the venturi), plus you've got fuel being added to the mixture.

2) Tuning / correct jetting

3) AVC (air cut valve) on carb that's supposed to enrichen the mixture on deceleration to prevent popping (doesn't always work so well)

4) Auto-decompressor system. The auto decompressor has been known to cause problems, but not too often. When it does, your bike will momentarily loose compression and it will fee like it's hesitating. You'll also feel a lack of throttle response. This has been known to happen when coming off WOT. If this is the cause, you can remove it from the camshaft, but you'll also have to weld up a hole on the cam to keep the top end properly lubed.

5) Also, this bike has a rev limiter that will kick in when at WOT under a load to prevent damage to the engine from over revving. If by chance what you're experiencing happens mostly at WOT and the bike feels like its cutting out, then perhaps you're bouncing off the rev limiter. Make note that the power on this bike falls off well below the rev limiter, so plan to shift before WOT.

By slowxr, Posted January 12, 2004

I'm running a 175 main with the stock airfilter.

The throttle response is fine everywhere else and the hesitation is only when backing off WOT.

The intake leak is very interesting because (I've only ridden the bike 3 times) I noticed dust in my airbox. I figured it had leaked past the airfilter. Perhaps I do have an intake leak and the dust is just another indicator or this.

By TimBrp, Posted January 13, 2004

I've had dust in my airbox too. I seal that sucker up with grease and all too. The aftermarket intake boot was tough to get on just right and I noticed that the clamp loosens with time. Check them out and tighten up if necessary. Also check the carb vent that goes under the seat, might be pinched.

By slowxr, Posted January 28, 2004

Here's my update. I changed out my 175 main and replaced it with a 172. No more hesitation! That was easy.